This invention relates in general to rotating devices and in particular to a method and apparatus for measuring the rotational rate of a rotating device.
Motor vehicles include an ever increasing number of electrical and electronic systems. Many such systems include components that can generate noise and vibration that can intrude into the vehicle passenger compartment and exceed the Noise, Vibration and Harshness (NVH) specifications for the vehicle. Anti-lock brake and vehicle stability systems include motor driven pumps and cyclically actuated solenoid valves that control application of pressurized brake fluid to wheel brake cylinders. A newer system being provided for vehicles is Electrically Powered Hydraulic Steering (EPHS), that utilizes a motor driven pump to provide hydraulic steering assistance. The EPHS system replaces the conventional hydraulic power steering system in which the power steering pump is mechanically belt driven by a pulley attached to the engine crankshaft. Use of an electric motor in place of the mechanical drive is more efficient since the pump speed is not a direct function of the engine speed. The motor speed varies as a driver requires steering assist, returning to a stand-by state if there is no assist needed.
Typically, a EPHS system includes a brushless DC motor that drives the pump for supplying pressurized hydraulic fluid. Such motors typically include a permanent magnet rotor having a plurality of poles mounted upon a motor shaft. The motor shaft and rotor are rotatably mounted within a stator having a plurality of windings. The motor also includes a switching circuit that sequentially energizes the stator windings to generate a rotating magnetic field. The rotating magnetic field urges the permanent magnet rotor poles into rotation.
The rotation of the motor rotor can generate noise and vibration. Should the motor be incorrectly assembled or include defective parts, such as bearings, the resulting noise and/or vibration may be excessive. Accordingly, it is desirable to test the motors under typical load conditions to assure that they are properly functioning and have been correctly assembled. Additionally, for the proper analysis of vibration, an accurate motor speed measurement is essential.
One known method for measuring motor speed consists of adding a tachometer generator to one end of the motor shaft. One such tachometer generator includes an annular tone ring mounted upon the end of the shaft. The tone ring formed from a low reluctance material, such as steel, and has a plurality of teeth extending from the circumference thereof. A transducer having a coil surrounding a permanent magnet is mounted within the motor housing adjacent to the tone ring. The permanent magnet generates a magnetic field that extends from the magnet pole adjacent to the tone ring toward the tone ring and then returns to the opposite magnet pole.
As a tone ring tooth approaches the magnet, the reluctance of the magnetic path decreases and as the tone ring tooth recedes from the magnet, the reluctance of the magnetic path increases. The magnet field varies inversely with the magnetic path reluctance. The varying magnetic field induces a voltage across the coil surrounding the magnet that is proportional to the rate of change of the magnetic field and generally proportional to the speed of the tone ring teeth passing the magnet. Thus, as the ring teeth sweep past the coil and magnet, a pulsed voltage is produced having a frequency that is a function of the motor speed. Accordingly, the resulting voltage frequency can be converted into the shaft speed. It will be appreciated that other methods are also available for measuring motor speed; however, they all require that additional components be included in the motor.
Alternately, it is possible to mount proximity sensors on the exterior of the motor. For example, one or more Hall effect devices can be mounted upon the outside of the motor housing. The Hall effect devices would detect the switching of field current within a reluctance motor. Again, the Hall effect devices would generate a pulsing voltage with the rate of the pulsed being a function of the motor shaft rotational speed.